Vascular and contractile responses to extracellular ATP: studies in the isolated rat heart

The dose-response characteristics for the effect of ATP upon cardiac function and vascular tone have been investigated in the isolated perfused rat heart. Vasodilation was observed with low ATP concentrations (0.01-0.1 mM) whereas severe vasoconstriction occurred with high concentrations (1.0-10.0 mM). At all doses studied, heart rate and pressure-rate product were reduced in a dose-dependent manner, with 10 mM ATP almost complete cardiac arrest was observed. Analysis of epicardial electrograms revealed that ATP induced arrhythmias, prolonged the P-R interval and induced partial blockade of S-A nodal activity and A-V conduction. Investigating possible mechanisms for the vascular and contractile effects of ATP, it was possible to exclude the calcium chelating properties of ATP and the effects of coincident ischemia arising as a consequence of ATP-induced vasoconstriction. Pharmacological studies revealed the ATP-induced vasoconstriction to be unresponsive to a range of coronary vasodilators and also allowed exclusion of prostaglandins, catecholamines and adrenergic receptors in the mediation of ATP effects. Investigations with acetylcholine revealed remarkably similar effects upon both contractile and vascular activity but studies with atropine suggested that the muscarinic receptor was not involved. Studies with theophylline allowed a dissociation of the vascular and contractile effects of ATP and indicated a possible involvement of the adenosine receptor in the cellular response to both high and low concentrations of ATP.